1. Field of the Invention
The present invention relates to techniques for reconstructing electron-beam (EB) images. More specifically, the invention relates to a technique for reconstructing an EB image using an inverse electro-optical calculation that corrects for an electro-optical transfer function.
2. Related Art
Electron beams (EBs) are widely used in micro-analysis and inspection. For example, an EB in a scanning electron microscope (SEM) may be used to measure a contour of a photo-mask or a wafer pattern with high spatial resolution. Typically, the resulting SEM image (which is more generally referred to as an ‘electron-beam image’ or an ‘EB image’) is often noisy or has a low signal-to-noise ratio (SNR) because a low beam current or rapid scanning is typically used to avoid damaging the photo-mask or the wafer. Alternatively, an EB in an EB-inspection device may result in an EB image with high SNR but lower spatial resolution than in an SEM.
However, in either of these applications, the resulting EB image corresponds to a convolution of the electro-optical transfer function of the measuring device (such as the SEM or the EB-inspection device) and the object being imaged or inspected (such as a photo-mask or a wafer). This additional contribution can degrade the quality of the results obtained using SEM imaging or EB inspection. In particular, in SEM imaging the electro-optical transfer function can increase the noise in the measured EB image, while in EB inspection the electro-optical transfer function can distort the measured EB image.
Hence, what is needed is a technique for reconstructing EB images that overcomes the problems listed above.